Increasing storage capacity is a goal in the data storage industry. Data storage products such as magnetic disk drives and optical disk drives, store digital information on rotating disks using a read/write head. The information is typically recorded in concentric, circumferential tracks about the near-center of the disk. One method to increase data storage capacity is to increase track density, where track density is defined as the number of tracks per radial inch (TPI) or in the metric system, number of tracks per millimeter radius.
Increasing track density involves many factors, some of which are reducing the read/write head's track width, increasing the resolution and frequency response of drive's head positioning system and increasing the signal to noise ratio (SNR) of the recording system. This patent addresses existing head positioning limitations by proposing a novel type of motor that is integrated into a suspension, head and HGA.
While the discussion below is focused on magnetic disk drives, it should be noted that the present invention also applies to optical disk drives, linear tape drives and other applications that require servo-controlled micro-actuation. The present invention additionally applies to the equipment used in the manufacture of disk drives and disk drive components. Two such components are disks and heads. Some of the equipment is known in the industry as disk certifiers, dynamic head testers and disk servo writers. An improved micro-actuated HGA with higher positioning resolution and higher frequency response improves reading and writing for all the equipment. It is particularly valuable to the disk servo writer that originally writes and defines for the disk drive the track location and density because improved written tracks and servo patterns increase drive performance.
Disk drives use a servo-controlled actuator to position the head on a desired track. The actuator moves the head from one track to another in a process called seek. It also maintains the head's position on a desired track in a process called track following. Track following is required because recorded tracks are not perfectly circular. Errors in the circular shape are due to spindle run-out, vibration modes from the numerous mechanical components of the disk drive, windage, acoustic noise and many interactions thereof. Tracking errors can be separated into repeatable run-out (RRO) and non-repeatable run-out (NRRO) components. The position error signal (PES), which is the position feedback signal from the head to the servo controller, has both low frequency and high frequency components. The servo controller operates up to the highest frequency possible while maintaining closed loop stability.
Most drives use a voice coil motor (VCM) to actuate head positioning. As track density increases, VCM actuators have limitations in resolution and frequency response. To increase storage capacity and increase track density, dual-stage actuators are being developed. The dual stage actuators use a VCM for seeking and coarse track following and use a micro-actuator for fine track positioning.
The use of the word motor and micro-actuator appear throughout this document. “Motor7” is a generic means for generating motion. “Micro-actuator” is a specific type of motor that generates motion for fine track positioning in disk drive and disk drive equipment. An “actuator” is a specific type of motor that generates motion for seeking and coarse track following in disk drive and disk drive equipment.
The previous art has three classifications of micro-actuators used in disk drives: suspension level, head level and collocation.
A suspension level micro-actuator has one or more micro-actuators integrated into the suspension's load beam. This design provides large displacements at the recording element because the long load beam creates mechanical advantage. These actuators suffer from insufficient frequency response at the head due to the low stiffness over the long distance between the micro-actuator and the head.
Head level micro-actuators have the micro-actuator integrated within the head. This design has the smallest distance between the actuator and recording element and therefore has very little mechanical loss. These designs suffer because they tend to distort the air bearing surface of the slider. With a head to disk spacing of say 10 to 15 nanometers, a distortion to the air-bearing surface (ABS) of a few nanometers is critical.
Collocated micro-actuators are mounted to the suspension flexure tongue and the head. The distance between the micro-actuator and head is small. The present invention promotes the use and benefits of collocated micro-actuators. However, collocation alone is insufficient to make a robust micro-actuator for disk drives.
To achieve desired performance, the mechanical stiffness between the microactuator and head must be maintained, micro-actuation must occur without inducing undesired vibrations in the head gimbal assembly (HGA), the displacement at the recording element must be sufficiently large and the frequency response of the entire system must be adequately high. Furthermore, the design must be capable of manufacture in terms of volume and cost, the structure must be robust to withstand in-process handling and cleaning, and the micro-actuator should minimally shed particles during drive operation. Finally, the micro-actuator must be able to operate in environmental extremes such as temperature, humidity, shock and vibration.
The objective of this invention is to improve upon these stated performance traits.
Three prior art patents are of particular interest because they have features utilized by the present invention. Although all three patents use collocated micro-actuators, the present invention is uniquely different from the prior art.
U.S. Pat. No. 6,246,552 B1 with date Jun. 12, 2001 and assignee TDK Corporation discusses a micro-actuated read/write head for disk drives. Micro-actuation is implemented using a piezoelectric or electrostrictive material that “elongates and contracts” which “linearly, circularly or rotationally” displaces the head for fine track positioning. The patent explains that internal vibration and unintended changes in flying height are problems that can be addressed. Although the present invention uses slider rotation for micro-actuation, it differs from the TDK patent at least because TDK does not use a shear mode piezoelectric micro-actuator.
U.S. Pat. No. 6,704,158 B2 with date Mar. 9, 2004 and assignee Western Digital Corporation discusses a head gimbal assembly for disk drives that uses a shear mode piezoelectric motor to micro-position the head with a “pure or nearly pure lateral movement.” Although the present invention uses a collocated, shear mode piezoelectric micro-actuator, it differs from the Western Digital patent at least because the present invention requires a rotational movement rather than a pure or nearly pure lateral movement.
U.S. Pat. No. 6,760,196 B1 with date Jul. 6, 2004 and assignee Western Digital discusses a micro-actuator with two offsetting hinges that rotates the slider. The patent states that the piezoelectric motor “expands” and “contracts” and references the TDK patent discussed above. This Western Digital patent does not use a shear mode piezoelectric micro-actuator and is therefore different from the present invention for at least that reason.
Unlike the three patents list above, this present invention is a rotational, shear mode, piezoelectric motor that is integrated into a collocated, rotational, shear mode piezoelectric micro-actuated suspension, head and HGA.
Rotating micro-actuators are now compared to transverse micro-actuators.
Transverse micro-actuators translate the head perpendicularly or laterally to the recorded track to achieve track following. The transverse micro-actuator exerts a force in one direction to the head and an equal and opposite force to the suspension. While the micro-actuator itself may be mechanically stiff and provide high frequency response, the suspension is not mechanically stiff on the transverse axis. Lack of stiffness reduces the frequency response of the system. The transverse micro-actuator pushes the entire mass of the head sideways yet has very little mass from which to push. Both mass and spring arguments reduce the frequency response of the transverse micro-actuator.
The rotating micro-actuator rotates the head about a rotational axis that is ideally located at the head's center of mass. In this ideal case, micro-actuating the head causes no net movement of the head mass along the transverse axis. Therefore, no mass-shifting disturbance is generated in the load beam and the low lateral stiffness of the suspension does not diminish the performance of the system. When the micro-actuator is activated, two equal torques are generated in the slider and flexure tongue. Because of the proximity and stiffness of these surfaces, high frequency response is achieved. While rotation about the exact center of mass of the head is improbable, it is understood that the closer the rotation axis is to the head's center of mass, the smaller a disturbance is created in the suspension.
Disk drive micro-actuators commonly use piezoelectric motors that utilize the transverse or longitudinal inverse piezoelectric effect. A control voltage applied to the micro-actuator causes the piezoelectric material to expand or contract creating a displacement. Usually in some form of beam shape, these piezoelectric motors have electrical and mechanical contact at the ends of the beam, allowing the middle section of the beam to freely expand and contract. The beam's ends are rigidly integrated with the rest of the micro-actuator system with conductive adhesive. Rigid integration is good for high frequency performance.
The longitudinal and transverse piezoelectric motors have several disadvantages when used as micro-actuators in disk drives. The middle section of the piezoelectric beam that is free to expand and contract is mechanically unsupported and therefore is less robust to shock and vibration. Mishandling or a large shock can fracture the brittle piezoelectric beam. Unsupported piezoelectric beams not only react to vibrations, they can also source and amplify vibrations. Another disadvantage arises when the desired axis expands and contracts, specifically, a secondary axis respectively contracts and expands. This secondary axis of actuation can lead to undesired displacement and vibration. These piezoelectric motors also have an undesirable property that moderate reverse voltages can depolarize the piezoelectric motor.
Micro-actuators that use shear mode piezoelectric motors have several advantages. Shear mode piezoelectric motors have highest displacement per volt of any piezoelectric configuration. Shear mode piezoelectric motors have a displacement that is independent of thickness, and therefore, can be thin and effective. Third, shear mode piezoelectric motors can be mechanically integrated into a micro-actuator system with far more stiffness and mechanical integrity than longitudinal and transverse piezoelectric motors. This feature is particularly important to the present invention.
Shear mode piezoelectric motors also avoid some of the disadvantages of the longitudinal and transverse mode piezoelectric motors. Shear mode piezoelectric motors allow bipolar operation, meaning they have equal displacements from applied positive and negative voltages. Shear mode piezoelectric motors avoid the reverse voltage depolarization property of the longitudinal and transverse piezoelectric motor types because shear mode piezoelectric motors operate with the electric field applied perpendicular to the polarization whereas the other types operate with the electric field applied parallel to the polarization.
The shear mode piezoelectric motor is oftentimes a thin, planar structure. To actuate, a voltage difference is applied to the top and bottom surfaces and the two surfaces move laterally (shear) with respect to one another. Because the entire area of the piezoelectric motor's top and bottom surfaces can be mechanically bonded into a microactuated assembly, the piezoelectric motor is stiffly integrated and highly supported along its entire length and width. Shear mode piezoelectric motors integrated into a microactuator can thus provide high frequency performance, minimal internal vibrations and improved robustness to external shock and vibration.
Prior art, discussed in the Fujitsu Scientific Journal December 2001 issue by author Shinji Koganezawa et al in a paper entitled “Development of a Shear Mode Piezoelectric Micro-actuator for Precise Head Positioning” explains how a shear mode piezoelectric motor is effectively integrated into a suspension level micro-actuator that increases microactuation frequency response and improves shock resistance.
FIG. 1 illustrates a disk drive 1 that has a base 2 to which a spindle motor 3 is attached. The spindle motor 3 rotates one or more disks 4 on which concentric circles of data are recorded one track 5 at a time by recording head 6. Head 6 is attached to a flexible suspension assembly 7 that is attached to a rigid E-block 8 that rotates about a pivot bearing 9. A voice coil motor 10 comprised of a voice coil rotor 11 attached to E-block 8 and a permanent magnet stator 12 attached to base 2 responds to a control voltage 13 that conducts through a HSA flexible circuit 14 from the circuit board 15 located beneath the base 2 (not visible) to position head 6 on a desired track 5.
FIG. 2 illustrates a head/gimbal assembly (HGA) 20. An HGA 20 is composed of a recording head 6 and suspension assembly 7. The head 6 is comprised of a read/write element 21 with head bond pads' 22 integrated on a ceramic slider 23 that has an air-bearing surface (ABS) 24. The suspension assembly 7 is comprised of a semi-rigid, typically stainless steel load beam 25, having a bend radius hinge 26, stiffening rails 27, an alignment hole 28, an alignment slot 29 and a half-sphere dimple 30 on which the head 6 will gimbal. Attached to the load beam 25 is a thick, stiff base plate 31 with a swage hole 32. Also attached to load beam 25, is a flexure assembly 33 comprised of a flexible stainless steel flexure 34 and flex circuit 35. The end of flexure assembly 33 located underneath the dimple 30 that attaches to head 6 is called the flexure tongue 36. Attached to flexure 34 is a flex circuit 35. Flex circuit 35 is composed of a top and bottom electrically non-conductive polyimide layer 41 with multiple, electrically conductive metal traces 42 sandwiched in between, that terminate at flexure tongue 36 with tongue bond pads 43 at one end, and at the other end, terminate near base plate 31 with base plate bond pads 44.
The load beam 25 and flexure 34 are typically manufactured from planar stainless steel sheets that are subsequently chemically etched to almost any two-dimensional (2D) design within the limitations of process tolerances. Typical etched features are holes, slots, beams and hinges. The present invention makes use of said 2D design features that can be routinely and precisely fabricated. The flat patterned stainless steel sheets are then formed into desired three-dimensional shapes.
The flex circuit 35 is generally made using thick and thin film technology. Almost any 2D shape can be created within the limitations of processing tolerances. It is common for the traces 42 of flex circuit 35 to bend around holes and slots, cross hinges, follow beams and flex around non-planar formed features. The present invention makes use of this ability.
FIG. 3A illustrates a prior art, passive longitudinal or transverse mode piezoelectric micro-actuator 50, which operates in the longitudinal or transverse inverse piezoelectric mode, that has a longitudinal or transverse mode piezoelectric beam 51, bonded with conductive adhesive 52 to a stationary support beam 53 on one side, and bonded with conductive adhesive 52 to a moving support beam 54 on the other side. The piezoelectric beam 51 has passive length 55 and passive thickness 56 when voltage 57 is applied across stationary support beam 53 and moving support beam 54 with a value of zero volts. Piezoelectric beam 51 has a polarization 58 that is parallel to the electric field 59 produced by applied voltage 57. Because the electric field 59 is zero, no elongation and contraction occurs.
FIG. 3B illustrates an active longitudinal or transverse mode piezoelectric microactuator 60 that is identical to said micro-actuator 50 except that voltage 57 now has a nonzero value. In the active state when a non-zero voltage 57 is applied, the polarization 58 and the electric field 59 are either parallel or anti-parallel and through the inverse piezoelectric effect, the piezoelectric beam 51 has either an elongated active length 62 and a decreased active thickness 63, or a shortened active length 62 and an increased active thickness 63 depending upon the polarity of voltage 57, as respectively compared to the passive length 55 and passive thickness 56.
There are several benefits of using longitudinal or transverse mode piezoelectric micro-actuator 60 in a disk drive 1 for micro-positioning head 6. The primary benefit is that active longitudinal or transverse mode piezoelectric micro-actuator 60 can micro-actuate moving support beam 54 by expanding and contracting freely over the length of unsupported beam 64 without sliding mechanical components that could generate undesired free particles and be a source of micro-contamination in disk drive 1. Also, piezoelectric beam 51 is bonded to and mechanically supported by stationary support beam 53 and moving support beam 54 with conductive adhesive 52 over adhesive length 65. Such mechanical integration by conductive adhesive 52 helps attain desired resolution and frequency response.
However several deficiencies arise when longitudinal or transverse mode piezoelectric micro-actuator 60 is used in a disk drive 1. The unsupported beam 64 is susceptible to deflection 66. Deflection 66 is caused by shock and vibration forces 67 external to the drive and by vibration sources internal to the drive such as the voice coil motor 10, the spindle motor 3 and said micro-actuator 60 itself. The piezoelectric beam 51 in deflection 66 can directly generate out of plane vibration modes. All vibrations get amplified or dampened by the various mechanical components of the disk drive 1 and a few high gain modes can limit the frequency response of the servo system. In general, it is desired that no out of plane vibrations are generated and that beam deflections are minimized.
Longitudinal or transverse mode piezoelectric micro-actuator 60 also has two limitations. The achievable displacement is less for negative voltages that than for positive voltages and therefore, said micro-actuator 60 cannot operate as a bipolar device at relatively high voltages. The negative voltage, and more appropriately the negative electric field strength, must be limited in magnitude to prevent polarization reduction or polarization reversal as determined by the piezoelectric material's electric field coercivity.
FIG. 4 illustrates a dual stage actuated disk drive 70 (prior art) that uses a suspension level micro-actuated HGA 71 as a secondary actuator. The spindle motor 3 rotates disk 4 on which track 5 is recorded by head 6. Head 6 is mechanically supported and positioned by a micro-actuated suspension 72, E-block 8, pivot bearing 9 and VCM 10. VCM 10 is the primary actuator that performs coarse positioning Fine track positioning is performed by micro-actuator 73. Micro-actuator 73 displaces head 6 along transverse axis 74. Transverse axis 74 is perpendicular to longitudinal axis 75, which is defined as being tangent to track 5 at read/write element 21 of head 6. Micro-actuator 73 works by expanding or contracting in push-pull fashion a pair of longitudinal or transverse mode piezoelectric beams 51 about hinge axis 76 causing a micro-actuator displacement 77 and a head displacement 78 along the transverse axis 74 of read/write element 21. Head 6 does not rotate about an internal axis, but rather, revolves a minute angle about hinge axis 76.
Mechanical advantage is the ratio of the head displacement 78 to the micro-actuator displacement 77. A suspension level micro-actuated HGA 71 has a large mechanical advantage (20× typical) due to the long distance of the load beam 25.
FIG. 5A illustrates in side view prior art from U.S. Pat. No. 6,760,196, of what the author of the present inventor defines as a “collocated, rotational, non-shear mode piezoelectric micro-actuator 80.” A load beam 25 with dimple 30 is attached to a flexure assembly 33 that has two offset hinges 82 (not visible in side view) that are attached by adhesive pads 83 and 84 to a longitudinal or transverse mode piezoelectric beam 51. Piezoelectric beam 51 also attaches to head 6 by adhesive pad 86. The piezoelectric beam 51 expands or contracts two offset hinges 82 which impart a rotation 87 of head 6 about rotation axis 88 as determined by the two offset hinges 82 located within the flexure tongue 36.
The design is “collocated” because the two longitudinal or transverse mode piezoelectric beams 51 are attached to flexure assembly 33 at the gimbal tongue 36 and to head 6. The design is “rotational” because a rotation 87 is imparted to head 6 about rotation axis 88. The design is identified as “non-shear mode” because a transverse or longitudinal mode piezoelectric beam 51 does not operate in shear mode.
FIG. 5B illustrates a magnified view of FIG. 5A that reveals in exaggerated fashion how the longitudinal or transverse mode piezoelectric beam 51 induces mechanical deformation at a microscopic level when a non-zero voltage 57 is applied. Because the adhesive pads 83 and 84 and the two offset hinges 82 impede the expansion of the piezoelectric beam 51 over the adhesive length 65 of adhesive pads 83 and 84, the piezoelectric beam 51, with its opposite side free to expand, bends at its ends as shown. Stated another way, the elastic properties of the stainless steel hinges 82 that are in contact with the longitudinal or transverse mode piezoelectric beam 51 are incapable of matching the high expansion rate of longitudinal or transverse mode piezoelectric beam 51. As a result, the unbalanced stress from expansion bends both the stainless steel hinges 82 and the piezoelectric beam 51. Similarly, the adhesive pad 86 and head 6 impede the expansion of the piezoelectric beam 51 over the adhesive length 65 of adhesive pad 86. With one side having impeded expansion and the opposite side free to expand, piezoelectric beam 51 has unbalanced stress and bends in the middle as shown. These microscopic bends in the piezoelectric beam 51 promote out of plane vibration. The build up of stress not only distorts the piezoelectric beam 51, it also distorts head 6. While shape deformation is generally undesired, deformation of the air bearing surface (ABS) 24 of head 6 is least tolerated.
The size of the adhesive pads 83, 84 and 86 has practical impact on the displacement performance of longitudinal or transverse mode piezoelectric beam 51. Large adhesive areas restrict the expansion and contraction of piezoelectric beam 51 but provide more mechanical stiffness to dampen unwanted vibrations. Small adhesive areas promote the free expansion and contraction of piezoelectric beam 51, yet the unsupported regions are more susceptible to unwanted vibration. Thus the tradeoff between maximizing displacement and maximizing vibration dampening is clearly identified for longitudinal or transverse mode micro-actuators 73.
FIG. 5C illustrates the same collocated, rotational, non-shear mode piezoelectric micro-actuator 80 as in FIG. 5A but now a shock force 89 that can be 600 G or more, such as due to head 6 slapping disk 4 when a disk drive 1 is accidentally dropped onto a concrete floor, causes a shock wave to pass through head 6, then through adhesive pad 86 to longitudinal or transverse mode piezoelectric beam 51. The necessary discontinuity between zones with and without mechanical support concentrates stress that facilitates fracture 90 of longitudinal or transverse mode piezoelectric beam 51.
FIG. 6 illustrates a prior art shear mode piezoelectric motor 100, which is a component of the present invention, where the shear mode piezoelectric material 101, typically a lead zirconium titanate (PZT) or any other shear mode piezoelectric material that is commercially available, has a top surface positive electrode 102, a bottom surface negative electrode 103 and a polarization 58. When voltage 57 is applied to positive electrode 102 and negative electrode 103, an electric field 59 perpendicular to polarization 58 in the shear mode piezoelectric material 101 causes a shear mode displacement 107 of the top surface positive electrode 102 relative to the bottom surface negative electrode 103 along a single axis that is parallel to polarization 58. If the applied voltage 57 is negative, then shear mode displacement 107 is negative. If the shear mode piezoelectric motor 100 is composed of a shear mode piezoelectric stack 108 and the electric field strength is held constant, then the shear mode displacement 107 is proportional to the number of layers of shear mode piezoelectric material 101 comprising the shear mode piezoelectric stack 108.
FIG. 7 illustrates prior art (U.S. Pat. No. 6,704,158) with an end view of a collocated, transverse, shear mode piezoelectric micro-actuator 110, where a shear mode piezoelectric motor 100 is attached to the flexure assembly 33 at the flexure tongue 36 and to the head 6 by an unknown adhesive 111. Ignoring how electrical contact to the shear mode piezoelectric motor 100 is implemented, a control voltage 13 causes a shear mode displacement 107 that equally displaces head 6 and recording element 21 along the transverse axis 74 to perform fine track positioning on track 5 of disk 4.
This end view clearly shows that the shear mode piezoelectric motor 100 moves the head 6 on the transverse axis 74, which is in agreement with the prior art patent claim of “pure or nearly pure lateral movement” of the slider 23.
Prior art U.S. Pat. No. 6,704,158 explains how micro-actuators that are collocated have a servo bandwidth typically greater than 5 kHz whereas micro-actuators that are not collocated (such as the micro-actuated suspension 72) typically have a servo bandwidth of 1.5 to 3 kHz. Prior art U.S. Pat. No. 6,704,158 explains how shear mode piezoelectric motor 100 has less friction and creates less particle contamination than longitudinal and transverse piezoelectric beams 51. The present invention takes advantage of these benefits.
This prior art is what the author calls a collocated, transverse, shear mode piezoelectric micro-actuator 110. It is collocated because the shear mode piezoelectric motor 100 is attached to flexure assembly 33 at the flexure tongue 36 and to head 6. It is transverse because the shear mode piezoelectric motor 100 moves laterally to track 5 on the transverse axis 74. It uses a shear mode piezoelectric motor 100 because it is explicitly stated in U.S. Pat. No. 6,704,158.
The second disadvantage is that a collocated, transverse displacement design does not provide mechanical advantage. The collocated, transverse, shear mode piezoelectric micro-actuator 110 has a mechanical advantage factor equal to one.
Rotational displacement does not have the two disadvantages of lateral displacement discussed above with respect to FIG. 7. Rotational displacement avoids the issue of mass displacement by rotating the head about its near center of mass. Rotational displacement is not limited to a mechanical advantage of one as will be discussed later.
The present invention uses a shear mode piezoelectric motor as a micro-actuator as does the Fujitsu prior art identified above; however, instead of using a suspension level micro-actuator, the present invention uses a collocated micro-actuator. A thin adhesive layer stiffly integrates the head, motor and suspension assembly to form a collocated, rotational, shear mode piezoelectric micro-actuated HGA.
An object of this invention is to provide a rotational, shear mode, piezoelectric motor that can be integrated in a collocated, rotational, shear mode piezoelectric microactuated suspension, head and HGA that produces rotational displacement for fine track positioning in disk drives and disk drive equipment.
An object of the invention is to minimize the mass of the motor.
An object of this invention is to create a rotational, shear mode, piezoelectric motor that directly generates rotational displacement without the need for hinges.
An object of this invention is to maximize mechanical contact of a collocated, rotational, shear mode, piezoelectric micro-actuator to the head and flexure assembly to increases stiffness, dampen vibration, manage stress and contain particles.
An object of this invention is to use the elastic properties of materials to make solid state structures that generate rotational displacement from shear mode piezoelectric motors that have linear displacement.